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><H1
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><A
NAME="AEN1949"
>8.3. More about shutdowns</A
></H1
><P
>It is important to follow the correct procedures when you shut
	down a Linux system.  If you fail do so, your filesystems probably
	will become trashed and the files probably will become scrambled.
	This is because Linux has a disk cache that won't write things
	to disk at once, but only at intervals.  This greatly improves
	performance but also means that if you just turn off the power
	at a whim the cache may hold a lot of data and that what is on
	the disk may not be a fully working filesystem (because only
	some things have been written to the disk).</P
><P
>Another reason against just flipping the power switch is that
	in a multi-tasking system there can be lots of things going on
	in the background, and shutting the power can be quite
	disastrous.  By using the proper shutdown sequence, you ensure
	that all background processes can save their data.</P
><P
>The command for properly shutting down a Linux system
	is <B
CLASS="COMMAND"
>shutdown</B
>.  It is usually used in one of
	two ways.</P
><P
>If you are running a system where you are the only user,
	the usual way of using <B
CLASS="COMMAND"
>shutdown</B
> is to quit
	all running programs, log out on all virtual consoles, log
	in as root on one of them (or stay logged in as root if you
	already are, but you should change to root's home directory or
	the root directory, to avoid problems with unmounting), then
	give the command <B
CLASS="COMMAND"
>shutdown -h now</B
> (substitute
	<TT
CLASS="LITERAL"
>now</TT
> with a plus sign and a number in minutes
	if you want a delay, though you usually don't on a single user
	system).</P
><P
>Alternatively, if your system has many users, use the command
	<B
CLASS="COMMAND"
>shutdown -h +time message</B
>, where 
<TT
CLASS="LITERAL"
>time</TT
>
	is the
	time in minutes until the system is halted, and 
<TT
CLASS="LITERAL"
>message</TT
>
	is a short explanation of why the system is shutting down.

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><PRE
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><TT
CLASS="PROMPT"
>#</TT
> <TT
CLASS="USERINPUT"
><B
>shutdown -h +10 'We will install a new 
disk.  System should
&#62; be back on-line in three hours.'</B
></TT
>
<TT
CLASS="PROMPT"
>#</TT
></PRE
></FONT
></TD
></TR
></TABLE
>

	This will warn everybody that the system will shut down in
	ten minutes, and that they'd better get lost or lose data.
	The warning is printed to every terminal on which someone is
	logged in, including all <B
CLASS="COMMAND"
>xterm</B
>s:

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><PRE
CLASS="SCREEN"
><TT
CLASS="COMPUTEROUTPUT"
>Broadcast message from root (ttyp0) Wed Aug  2 01:03:25 1995...

We will install a new disk.  System should
be back on-line in three hours.
The system is going DOWN for system halt in 10 minutes !!</TT
></PRE
></FONT
></TD
></TR
></TABLE
>

	The warning is automatically repeated a few times before the boot,
	with shorter and shorter intervals as the time runs out.</P
><P
>When the real shutting down starts after any delays, all
	filesystems (except the root one) are unmounted, user processes
	(if anybody is still logged in) are killed, daemons are shut down,
	all filesystem are unmounted, and generally everything settles
	down.  When that is done, <B
CLASS="COMMAND"
>init</B
> prints out a
	message that you can power down the machine.  Then, and only then,
	should you move your fingers towards the power switch.</P
><P
>Sometimes, although rarely on any good system, it is
	impossible to shut down properly.  For instance, if the kernel
	panics and crashes and burns and generally misbehaves, it might
	be completely impossible to give any new commands, hence shutting
	down properly is somewhat difficult, and just about everything
	you can do is hope that nothing has been too severely damaged
	and turn off the power.  If the troubles are a bit less severe
	(say, somebody hit your keyboard with an axe), and the kernel
	and the <B
CLASS="COMMAND"
>update</B
> program still run normally,
	it is probably a good idea to wait a couple of minutes to give
	<B
CLASS="COMMAND"
>update</B
> a chance to flush the buffer cache,
	and only cut the power after that.</P
><P
>Some people like to shut down using the command
	<B
CLASS="COMMAND"
>sync</B
>
	
		<A
NAME="AEN1977"
HREF="#FTN.AEN1977"
>[1]</A
>
		
	three times, waiting for the disk I/O to stop, then turn off
	the power.  If there are no running programs, this is about
	equivalent to using <B
CLASS="COMMAND"
>shutdown</B
>.  However, it
	does not unmount any filesystems and this can lead to problems
	with the ext2fs ``clean filesystem'' flag.  The triple-sync
	method is <EM
>not recommended</EM
>.</P
><P
>(In case you're wondering: the reason for three syncs is
	that in the early days of UNIX, when the commands were
	typed separately, that usually gave sufficient time for most
	disk I/O to be finished.)
	</P
></DIV
><H3
CLASS="FOOTNOTES"
>Notes</H3
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><A
NAME="FTN.AEN1977"
HREF="x1949.html#AEN1977"
>[1]</A
></TD
><TD
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WIDTH="95%"
><P
><B
CLASS="COMMAND"
>sync</B
> flushes the
		buffer cache.  </P
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